U.S. Pat. document No. 3,744,301 discloses a device for indicating the level of a liquid in a tank, which device comprises an acoustic wave transmission line placed vertically in a receptacle containing a liquid, the line having a transmitter transducer at a first end for creating a train of elastic waves in the transmission line, and having a second transducer at a second end for delivering a signal in response to the train of elastic waves which is attenuated as a function of the quantity of liquid in contact with the transmission line.
Such a device is difficult to implement since the transducers which are disposed inside the receptacle containing the liquid must be provided with means for protecting them so as to avoid being damaged by the liquid. Further, connections between the transducers and the electronic processing circuits must pass through the walls of the receptacle at the transmitter transducer and at the receiver transducer.
A variant of the above device is known in which the transducers are placed at two opposite ends of the transmission line, and the line and the transducers are disposed substantially in a single horizontal plane. In this case, only one opening need be formed through the side wall of the receptacle in order to mount the device for detecting the level of the liquid. However, given that the transmission line and transducer assembly must be disposed inside the liquid-containing receptacle, the sensor occupies considerable volume inside the receptacle. Further, the sensor includes recesses in which waste or sludge may collect, and this can affect the reliability of the system.
United Kingdom patent documents GB-A-2 137 348 and GB-A-2 076 536 disclose elastic wave sensors in which the transmitter and receiver transducer devices are disposed directly on the wall of a liquid-containing receptacle, or they are coupled to said wall, and they are used to detect the presence of a liquid at a predetermined level in the receptacle. In this case however, the presence or the absence of liquid is determined by detecting an attenuation in the amplitude of the signals received by the receiver transducer, and this makes the device relatively insensitive and unreliable.
The present invention seeks to remedy the above-mentioned drawbacks and to provide a device for detecting the presence of a liquid, said device being simple to implement and making it possible to determine reliably whether a given level has been reached by the liquid in a tank.
The invention seeks in particular to provide a device which is compact and can easily be mounted on a tank such as a vat, which has a good sensitivity, and which is reliable even if the inside of the vat containing the liquid whose level is to be detected contains an atmosphere which is very corrosive, or if its temperature is very high, or if the liquid is carrying waste, sludge, or other solid particles.
The invention also seeks to provide a device which does not require openings to be formed through the tank in the vicinity of the level to be monitored.